The invention is applied in the technical sector for the recycling of treated wood and in particular those treated with CCA (copper, chrome, arsenic) which are the most difficult to recycle due to the arsenic contained in them and which are used in numerous applications, notably for power and telephone line posts as well as railway sleepers, pallets, etc.
The recycling of wood-based products is presently undertaken to a very limited degree, to comply with environmental protection requirements and regulations as well as for economic reasons.
The recycling of untreated wood poses no serious problems as the composition products for these materials may be burnt or even shredded, ground for use in the composition of other natural sub-products or for mixture with other plastic components, etc.
The recycling of treated wood is more difficult as it presents problems for which presently no solution has been found. The recycling has been made obligatory in the EC by decision of the European Council on Dec. 22, 1994 and application of Directive EC 91/689 of Dec. 12, 1991.
If we consider the application of treated wood in the manufacture of electrical or telephone posts and railway sleepers, the extent of this problem becomes clear bearing in mind how many million posts and sleepers are in use in France and abroad. These posts and sleepers have an average working life of approximately 25 years. The treatment of these posts is generally carried out using a vacuum impregnation process. In France, posts have generally been treated with CCA for the last 25 years as this treatment offers far better results, but also, and foremost, for its antiseptic qualities, cleanliness and longevity which remain unequalled today thanks to CCA's ability to fix itself to the wood which is far superior to that of all other existing wood protection products.
One of the problems encountered resides in the fact that it is practically impossible for an uninformed person to tell the difference between wood treated with metallic salts and in particular those treated with CCA and other old untreated wood, whereas wood treated with creosote or pentachlorphenol is easily recognisable due to its exterior aspect.
Furthermore, the burning of treated wood may be extremely dangerous and even more so when the wood has been treated with CCA and this not only in respect to the possible environmental pollution but also where the health of persons is concerned.
Bearing in mind the fact that treated and untreated wood from posts and sleepers is collected in an empirical and disorganised manner, one easily understands the need, in respect to public safety, for a rational structure for the gathering of old treated wood and its later elimination or recycling.
Various attempts have been made to solve the problem posed by recycling of treated wood in general and in particular that treated with CCA.
First of all it was suggested that the wood be reused but this proved of little economic interest and its commercialisation was not feasible under satisfactory economic conditions.
Another possibility was to recycle it as decorative elements in flower beds by grinding the wood and then treating it with colouring in an autoclave. This technique was not followed up due to the lack of possible outlets for the product.
The solution of dumping the wood in landfills is also not possible due to legal restrictions.
Storage may hardly be considered a solution due to the space requirements.
Burning the wood in traditional incinerators together with other waste is also not possible as the elements contained in the treated wood, in particular the arsenic in CCA, tend to combine with the other elements present in the waste to form composites which cannot be controlled. Furthermore, due to the effect of the thermal shock between ambient air and incinerator temperature, most of the arsenic contained in the wood treated with CCA evaporates and takes with it other heavy metals.
Above 450.degree., the arsenic trioxide particles become extremely fine and difficult to trap except using considerable means for the capture and washing of smoke. Furthermore, a more serious situation may arise in which the other elements burnt together with the wood treated with CCA may produce nascent hydrogen which when combined with the arsenic of CCA, forms arsine.
Based on the results of these experiments and taking into consideration the associated restrictions, it was decided to employ other recycling methods in order to firstly separate the wood from the treatment product. To achieve this, various separation systems such as chemical washing, microbiology and controlled combustion were studied.
The chemical washing method consists of separating the heavy metals with humidity, by first grinding the wood as fine as possible and then immersing it in a bath containing an acid solution. A filter is then used to separate the liquid from the humid dust which is then considered clean and may be processed in any combustion installation without risk of pollution. However, this technique has several inconveniences.
Regardless of the chemical mixture used, it always contains acids and the volumes required are always enormous, irrelevant of the concentration in the solution.
This technique uses a cascade system in which, after having washed the wood in an acid solution, the acid solution must be washed in water and the water must then be treated before reuse. This increases the volume to be treated and the size of the installation.
Furthermore, the cycle times are long, the installation size is considerable and the investment costs are high.
Finally, such a system is selective and may only be used with one specific type of treatment product at a time.
Microbiological separation is still at an early stage and offers no industrial application to solve the problem at hand. Studies have been carried out on different stocks of mushrooms but are presently still in the experimental stage.
Numerous studies have been carried out in respect to combustion separation in order to determine whether or not arsine is formed and how the arsenic behaves in wood treated with CCA.
If, in the first case, the authors and experts agree and consider that there is no risk of arsine formation when wood treated with CCA is burnt alone, the contrary is true in the second case. As the experimental procedures employed differ greatly, the results also differ to such an extent that in most of the countries concerned the administrative authorities have reacted in a similar manner and have taken the "wait and see" solution: temporary storage or burial of old treated wood.
Following this, most of the teams working on the combustion of treated wood solution have given up. Nonetheless, in view of current knowledge and the costs and investments necessary, only the separation by combustion method may realistically be adopted today to enable the industrialisation of the recycling of treated wood.
The incineration of wood in a special reactor at temperatures above 1,300.degree. Celsius in order to volatilize the whole and then to recuperate all the products contained in the combustion gas using a complex installation, is a feasible but nonetheless costly solution. The risks associated with this technique and the handling of gas containing heavy metals at a high temperature are considerable. This requires the implementation of safety systems and facilities as well as numerous reliable inspection procedures with a continuous maintenance program. Furthermore, the maintaining of such a high temperature requires large quantities of energy and in all cases more energy than could be recuperated from the combustion of the wood. The considerable investment necessary for the implementation of safety systems adapted to this method require the construction of enormous installations in order to bring these costs into proportion. The need for such an enormous installation, however, makes the operation of such a system quite unrealistic.
It is therefore based on these restrictions, various work undertaken and mentioned here, that the patent applicant has undertaken to find a solution for the problem in question and in particular for posts treated with CCA, of which he is one of the major manufacturers.